Bump sphinx

Bumps the pip-dependencies group with 1 update in the /third_party directory: [sphinx](https://github.com/sphinx-doc/sphinx).


Updates `sphinx` from 8.0.2 to 8.1.3
- [Release notes](https://github.com/sphinx-doc/sphinx/releases)
- [Changelog](https://github.com/sphinx-doc/sphinx/blob/master/CHANGES.rst)
- [Commits](sphinx-doc/sphinx@v8.0.2...v8.1.3)

---
updated-dependencies:
- dependency-name: sphinx
  dependency-type: direct:production
  update-type: version-update:semver-minor
  dependency-group: pip-dependencies
...

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]

third_party/requirements.txt

@@ -13,5 +13,5 @@ sphinxcontrib_htmlhelp==2.1.0
 sphinxcontrib_serializinghtml==2.0.0
 sphinxcontrib_qthelp==2.0.0
 breathe==4.35.0
-sphinx==8.0.2
+sphinx==8.1.3
 sphinx_book_theme==1.1.3